The present invention relates to a laser beam receiver and, more particularly, to a laser beam receiver provided with a filter member capable of expanding a laser beam in a scanning direction and of diffusing the laser beam.
A laser beam receiver for receiving a laser beam emitted by a rotary laser device uses a conventional filter for detecting a laser beam focused in a sufficiently large spot to determine the position of the laser beam. The filter is capable of transmitting light of a necessary wavelength and of cutting light of unnecessary wavelengths to the best of its ability.
When a rotary laser device which emits a laser beam of visible light is used, the laser beam can be visually recognized and hence the laser beam must be focused in a spot of the least possible diameter to improve the visibility of the laser beam.
In particular, a rotary laser device for determining a reference position for altitude measurement includes a rotary laser unit, and a light position determining unit which receives a laser beam emitted by the rotary laser unit and determines a reference position for altitude measurement. The rotary laser used an invisible laser beam because a laser light source that emits a visible laser beam was not available.
When an invisible laser beam is used, a light position determining unit is necessary because the laser beam is invisible. The light position determining unit includes a photodetecting device provided with a plurality of photodetectors. The position of the laser beam is determined from the output signal ratio between the output signals of the photodetectors. Since the output signal ratio is used, the laser beam does not need to be focused in a pinpoint spot on the photodetecting device and it is more effective to focus the laser beam in a spot covering some photodetectors.
When a visible-light emitting laser is used, a position on which a laser beam falls can be visually recognized. However, a laser having a limited power capacity must be used to avoid injuring eyes with the laser beam. Since the laser beam emitted by the laser having a limited power capacity has limited energy, the laser beam must be focused in a pinpoint spot having a high luminance and the collimation of the same must be enhanced. However, since it is difficult to collimate the laser beam in the field, a light position determining unit is necessary.
The aforesaid conventional rotary laser device, however, is unable to detect the small spot of the laser beam by the light position determining unit in the worst case where the spot of the laser beam is smaller than gaps between the photodetectors (PDs) of the light position determining unit. Even if the spot has a diameter substantially equal to a lower limit diameter that enables the detection of the spot of the laser beam, the position of the light cannot be measured satisfactorily (in a satisfactory resolution).
It is prevalent to arrange two photodetectors, i.e., an upper photodetector PD1 and a lower photodetector PD2, of the light position determining unit in a direction perpendicular to the scanning direction of the laser beam as shown in FIGS. 6(a) and 6(b). A position where the respective outputs of the photodetectors PD1 and PD2 are the same is the position of the laser beam. The laser beam can be detected when the spot of the laser beam is sufficiently large as shown in FIG. 6(a). However, if the spot of the laser beam is smaller than the gap between the photodetectors PD1 and PD2 as shown in FIG. 6(b), the laser beam cannot be detected.
If the light position determining unit is provided with triangular photodetectors PD1 and PD2 arranged as shown in FIG. 7, the difference between the respective outputs of the photodetectors PD1 and PD2 cannot be obtained and the position of the laser beam cannot be determined unless the laser beam is focused in a spot having a large dimension in the scanning direction. When the triangular photodetectors PD1 and PD2 are arranged as shown in FIG. 7, the position of the spot on the photodetector can be determined on the basis of a peak voltage proportional to the area of part crossed by the spot. Therefore, the difference between the respective outputs of the photodetectors PD1 and PD2 cannot be measured and the position of the laser cannot be determined unless the laser is focused in a spot having a large dimension in the scanning direction.
When the laser beam scanning the photodetector is focused in a small spot, the output of the photodetector is dependent on the energy of the laser beam and is not dependent on the area and hence the peak voltage is the same regardless of the position of the spot on the photodetector. Therefore a peak voltages provided by the photodetector scanned with a laser beam {circle around (1)} and a peak voltage provided by the photodetector scanned with a laser beam {circle around (2)} are the same and hence scanning position cannot be determined. The gap between the photodetectors cannot receive the laser beam.
FIGS. 8(a) and 8(b) show a light position determining unit provided with two photodetectors respectively having different areas. FIG. 8(a) illustrates scanning with a laser beam focused in a large spot and FIG. 8(b) illustrates scanning with a laser beam focused in a small spot. When the small spot is used as shown in FIG. 8(b), the difference between the respective outputs of photodetectors PD1 and PD2 cannot be obtained and hence the position of the laser beam cannot be determined.
The photodetectors PD1 and PD2 shown in FIGS. 8(a) and 8(b) have different areas, respectively. The mode of detection of the laser light by the arrangement shown in FIGS. 8(a) and 8(b) is basically the same as the mode of detection of the laser light by the arrangement shown in FIG. 7. FIG. 8(a) shows a case where the spot of the laser beam is sufficiently large as compared with the photodetectors. The magnitude of peak voltage is dependent on the position of the spot on the photodetector. FIG. 8(b) shows a case where the spot of the laser beam is small. In this case, the peak voltage is constant, the difference between the respective outputs of the photodetectors PD1 and PD2 cannot be obtained and hence scanning position cannot be determined.
According to the present invention, a laser beam receiver for receiving a scanning laser beam includes a filter device, and a photodetecting means provided with a plurality of photodetectors, wherein the filter device has a lenticular part capable of expanding a scanning laser beam in a scanning direction, and a diffusing part capable of diffusing the laser beam transmitted by the lenticular part.